In the 26 years since the first offshore wind farm was installed in Vindeby, Denmark, the offshore wind industry has undergone remarkable growth. Recently, this growth has accelerated, advancing farms rapidly into deeper water and utilizing larger turbines. As offshore wind farm developers continue to look towards heavier, higher capacity turbines, and harsher sites, improving cost-effectiveness will rely increasingly on support structure optimization. Monopile foundations remain the preferred option, but the “XL” monopiles required in deeper water are sensitive to wave-induced fatigue loads and exceed the weight capacity of many installation vessels. The goal of the Hybrid Monopile support structure, is to provide a cost-effective alternative to the monopile in water depths greater than 30 meters. By providing an open structure in the key wave loading area of the water column, wave-induced fatigue damage can be reduced, allowing for a thinner monopile shell and a substantially lighter support structure. Design of the Hybrid Monopile began with the optimization of the brace member length and diameter for a specific location and turbine in the North Sea. Computations. were carried out using a Matlab-based model in which the structure is represented as a discretized Euler-Bernoulli beam. Extreme load cases were applied in a static analysis to ensure that selected dimensions are able to resist buckling of either the monopile shell, or the brace members. An accompanying finite-element model of the structure was used to verify that the selected brace dimensions were sufficient under extreme loads. To estimate the fatigue life of the monopile shell, and the brace members, wave loads were handled in the frequency domain and wind loads in the time domain. Damage equivalent loads (DELs) for each were then combined using quadratic superposition. Wind and wave roses for the selected site were used in the fatigue analysis to capture the effects of misaligned wind-wave cases on the calculated fatigue life. In this design phase, the Hybrid Monopile was shown to experience sixty percent less wave-induced fatigue damage than the traditional monopile. This baseline version of the design was then modified for 256 different combinations of water depth, turbine size, wave climate, and monopile diameter, to demonstrate the versatility of the concept and identify the design drivers. During the design process, the Hybrid Monopile was updated by altering the monopile shell thickness, brace diameter, and brace length. In each case, a traditional monopile was also designed in parallel, to serve as a basis for comparison. Optimization of each configuration was based on an iterative series of design checks during which the variable design dimensions were increased or decreased based on the structure natural frequency, response to an extreme load, or expected fatigue life at one of the critical locations; the mudline and the individual brace cross sections. Results of the iteration showed that in more than 3/4 of the test cases, which covered water depths from 30-60 meters, and turbine sizes of 8-20 MW, a Hybrid Monopile could be established that offered lower structure weight and improved fatigue performance at an equal or reduced cost compared to a traditional monopile. Viable configurations showed average weight and cost reductions of 27% and 17% respectively. These results were used to establish a limited number of standardized Hybrid Monopile classes, which could be mass produced and assigned to a wind farm, based on only the water depth and turbine size. Looking forward, further work is required to determine the optimum connection detail at the brace-monopile interface, and to assess the fatigue risk at that junction.

The increase in demand for renewable energy has resulted in higher demand for wind energy. To meet this requirement, the wind turbine sizes are increasing rapidly, and this results in increasing load on the connections between MP-TP and the segments of the tower. This research focuses on traditional L-flange connection and novel C1 wedge connection, former the most widely used in the offshore industry and other being new to the offshore industry. The objective of this thesis is to evaluate the limits of the L-flange and C1 wedge connection. The design of the L-flange connection carried out on the basis of Petersen’s theory[1]. It is designed to have higher ULS resistance with steel mass as low as possible. This analytical designed is then compared to finite element analysis (FEA). Previous study of Cheng[2] is used to validate the setup and methodology for FEM in Ansys. For the same overturning moment, C1 wedge connection is designed using the design tool provided by C1 Connections. The design check for flanges is carried out. The design is then compared to finite element analysis (FEA). Fatigue limit state was verified for both the connections. Based on the study, the following conclusions are made. Firstly, L-flange connection has around 30% higher ULS resistance compared to design overturning moment of 609MNm whereas C1 wedge connection has 55% higher ULS resistance. Secondly, L-flange connection has higher meridional deformation at the same elevation in the shell as compared to C1 wedge connection. The gap opening at the interface of the flanges is studied and it is observed that C1 wedge connection opens after the loss of contact force at the interface which is generated more efficiently through the pretension of stud whereas L-flange connection being an eccentric connection starts opening without full loss of contact force at the interface and the secondary path of load transfer is activated. This results in the lower meridional rigidity provided by the L-flange connection as compared to the C1 wedge connection. Lastly, C1 wedge connection provided an opportunity for several optimizations to have the same ULS resistance as that of L-flange connection. The mass of steel reduced for these optimizations. The L-flange connection weighed 53.72 tonnes for both ULS and FLS criterion whereas C1 wedge connection weights 20.8 tonnes for ULS criterion and 28.8 tonnes for FLS criterion.

The field of wind energy has experienced significant expansion and development driven by the need to reduce reliance on fossil fuels. Offshore wind turbines have become increasingly popular leading to larger turbines with greater energy output to match the growing demand. The most common foundation type of offshore wind turbines is the monopile foundation, consisting of two large diameter steel tubulars, the monopile (MP) and the transition piece (TP). As wind turbine generators (WTGs) increase in size, the transportation and installation of MP foundations becomes more complex and expensive, requiring specialized vessels with adequate lifting capacity. This research investigates a new connection – the underwater wedge connection – as a method of connecting an MP to a TP underwater. By having an underwater connection, the size of the MP can be reduced as it no longer has to exceed the waterline. The wedge connection consists of a dowel with an inclined plane and two flanges. A number of dowels will be placed around the circumference of the connection, fitted onto flanges on both the MP and TP. As the dowels are pushed into position, the inclined plane creates a vertical preload between the two flanges. This allows a tensile load on a segment of the connection (caused by the bending moment in the foundation) to be transferred to the foundation via two load paths: reduction of the preload, and by loading the dowel itself in shear. First an analysis on the current state of art and relevant design codes was carried out. This analysis highlighted the requirements the connection needed to satisfy, in order to serve as starting points to make decisions regarding the design. The preliminary design of the connection is made using analytical calculations. The structural integrity of the flanges and the dowel at the Ultimate Limit State (ULS) was verified. A 3D model of the connection was then created to perform numerical analyses using ANSYS Static Structural. The behaviour of the connection under ULS and Fatigue Limit State (FLS) loading is studied. The opening and failure point of the connection along with the sensitivity of various parameters are also investigated. The results show that connection was able to effectively transfer both tension and compression loads. The connection has a mechanical advantage of 1.95, meaning that it achieves the same preload as the ULS load by applying only 51.3% of the ULS segment load during installation. The connection opens gradually and only opens at a load higher than the ULS load. Even after opening, it continues to transmit loads effectively, with ultimate failure governed by the yielding of the lower flange. Additionally, the connection exhibits good fatigue resistance, with a low fatigue damage level of 3.8%. It is recommended to conduct experiments to validate the numerical model employed in this study. Further studies should also be carried out to investigate the impact of structural imperfections on the behaviour of the connection.

The hybrid monopile is a substructure concept for offshore wind turbines initially designed and developed by Fistuca B.V. This Thesis is about the initial concept design of the connection details in the hybrid monopile. Several concept connections are designed. The best concept is selected and thereafter assessed on the ultimate limit state, serviceability limit state and the fatigue limit state. This is all done using the finite element program ANSYS APDL.

Wind turbines constitute a sustainable and effective solution for the production of energy using wind power. Offshore wind turbines are becoming of special interest. However, their design poses great challenges, since an offshore structure is subjected to combined wind and wave dynamic loading that is characteristic of the installation site and play an important role on the fatigue failure of the wind turbine structure. One part of specific interest for the fatigue assessment of a wind turbine is the way that the transition piece is connected to monopile. Since now, grouted and bolted connections are commonly used, but their many drawbacks regarding their performance inspired Fistuca BV to invent a new type of connection, the wedge connection, in order to increase the fatigue life of the wind turbine structures and decrease simultaneously their construction and installation cost. More specifically, a basic flange with inclined planes is attached on the top of the monopile and a more complex one (fork-shaped flange) is fitted to the bottom of the transition piece. Inclined dowels are used to connect the two flanges and secure the structure. The final positioning of the dowels is achieved by applying external pressure on the back side of them, which is converted to a vertical reaction force between the two flanges through the inclined planes, acting as preload. The work that will be presented here deals with the fatigue assessment of the wedge connection, and how the presence of the preloading force may affect the fatigue life of it. A reference wind turbine and a reference location have been selected first, in order to calculate all the fatigue loads resulted on the structure due to wind and wave at the level where the wedge connection has been selected to be located (+4m above the MSL). The concept of damage equivalent loads has been applied. Damage loads due to waves have been calculated in the frequency domain, while damage loads due to wind have been handled in the time domain due to strongest non-linearities in the rotor aerodynamics. The process of damage loads’ calculation was carried out using MATLAB. Once the loads have been calculated, the 3D CAD design software of SOLIDWORKS is used to design all the components of the wedge connection and assemble them to their final position. To facilitate the design process and reduce the computational time, the flange connection is reduced to a single wedge segment, on which the highest stresses are developed. Two load cases are examined for each one of the above resulted damage loads: one with preload and another one without preload. For the purpose of this thesis, xx MPa external pressure has been applied for the final positioning of dowels in case of preload. ANSYS Workbench has been used for the finite element analysis. Nominal stresses are calculated and based on selected S-N curves for both DNV-GL codes and Eurocode 3 and by making use of the Miner’s rule, the damage and the fatigue life of the connection is calculated. The selection of the appropriate S-N curve for each load case and for each component of the connection, has been based on results from test specimens. A parametric study has been carried out as well, in order to check several parameters that may affect both the accuracy of the finite element analysis and the stress development on the connection, and as a result the damage and the fatigue life of it.

More offshore wind projects are being developed. Shallow locations are more occupied and a tendency to deeper water and to larger wind turbines is seen. These create more bending moment which can be more easily resisted by jacket substructures. The state of the art installation process of a jacket is as follows. The foundation piles are driven into the seabed. The jacket leg is installed by sticking a stabbing cone into the stick-up length of the foundation pile and fill the space between the cone and the pile with grout. In this situation a lot of grout, steel and installation time are used. An alternative connection could be a wedge connection which is considered in this thesis. A wedge connection consists of four parts: jacket leg, connector element, wedges and foundation pile. The challenge to apply this connection to jackets is to overcome the installation tolerances. Due to the inaccuracy of the driving of the foundation piles, the holes in the jacket legs and the foundation piles do not align and the wedges cannot be installed. The installation tolerances consist mainly of center-to-center distance, pile verticality and a vertical tolerance. The maximum translational tolerances are 75 millimeter while the maximum rotational tolerance is maximum 0.83 degrees. 23 different concepts have been developed to deal with the installation tolerances. All the concepts have some drawbacks and do not fully solve the problem without downsides. The magnitude of the installation tolerances is therefore reduced and from a multi-criteria analysis a screw thread connection is proposed. The advantages of the screw thread connection are: a reduced amount of material, no grout is required and the installation time of the jacket to foundation pile connection can be reduced up to 75 percent. The screw thread connection overcomes the vertical tolerance and the tolerance around the longitudinal axis of the foundation pile by rotation of the connector element. In case of an pile inclination, the connector element and the foundation pile do not fully align and a gap remains in between them. By applying a preloading force to the connection it is still not possible to close this gap. The space between the connector element and the foundation pile has to be filled with an epoxy resin. The screw thread connection does fulfill the requirements for the ultimate limit state based upon hand calculations for net stresses, bending stresses and contact stresses. The fatigue limit state of the wedge connection meets the requirements. Only the fatigue limit state of the screw thread does not fulfill the requirements based upon a hand calculation. The three largest issues of the screw thread connection are the fabrication of the screw thread profile, the fatigue in the screw thread and the installation of the epoxy resin between the connector and the foundation pile. The first step to develop the screw thread connection further is an more extensive investigation to these three issues.